The present invention relates to a method for reducing the solids content of a waste water retention pond associated with aqueous methods of extracting bitumen from tar sands.
Tar sands, which are also known as oil and bituminous sands, are siliceous materials which are impregnated with a heavy petroleum. The largest and most important deposits of the sands are the Athabasca sands, found in northern Alberta, Canada. These sands underlay more than 13,000 square miles at a depth of 0 to 2,000 feet. Total recoverable reserves after extraction and processing are estimated at more than 300 billion barrels. Tar sands are primarily silica, having closely associated therewith an oil film which varies from about 5 to 21percent by weight, with a typical content of 13 weight percent of the sand. The oil is quite viscous - 6 to 10.degree. API gravity -- and contains typically 4.5% sulfur and 38% aromatics. In addition to the oil and sand components, tar sands contain clay and silt in quantities of from 1 to 50 weight percent, more usually 10 to 30%. The sands also contain a small amount of water, in quantities of 1 to 10% by weight, in the form of a film around the sand grains.
Several basic extraction methods have been known for many years for the separation of oil from the sands. In the so-called "cold water" method, the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water, water with a surface agent added, or a solution of a neutral salt in water, which salt is capable of acting as an electrolyte. The combined mass is then subjected to a pressure or gravity separation.
In the hot water method, as disclosed in Canadian Pat. No. 841,581 issued May 12, 1970, the bituminous sands are jetted with steam and mulled with a minor amount of hot water at temperatures of 170.degree. to 190.degree. F., and the resulting pulp is then dropped into a turbulent stream of circulating hot water and carried to a separation cell maintained at a temperature of about 185.degree. F. In the separation cell, sand settles to the bottom as tailings and oil rises to the top in the form of a froth. An aqueous middlings layer comprising clay and silt and some oil is formed between these layers. This basic process may be combined with a scavenger step for further treatment of the middlings layer obtained from the primary separation step to recover additional amounts of oil therefrom.
The middlings layer, either as it is recovered from the primary process or as it is recovered after the scavenger step, comprises water, clay and oil. The oil content is, of course, higher in middlings which have not undergone secondary scavenger steps. In the hot water extraction process as mentioned above, waste water streams are removed from the process plant as a slurry of about 35 to 75%, solids by weight. Included in the slurry is sand, silt, clay and small quantities of bitumen.
In this specification, sand is siliceous material which will not pass a 325 mesh screen. Silt will pass 325 mesh and is smaller than 45 microns but is larger than 2 microns. Clay is material smaller than two microns including some siliceous materials of that size.
Because this waste water contains oil emulsions, finely dispersed clay with poor settling characteristics and other contaminents, water pollution considerations prohibit discarding the effluent into rivers, lakes or other natural bodies of water. The disposal of the waste streams has therefore presented a problem.
Currently, waste water is stored in retention ponds which involve large space requirements and the construction of expensive enclosure dikes. A portion of the water in the waste water stream can be recycled back into the hot water extraction process as an economic measure to conserve both heat and water. However, experience has shown that the dispersed silt and clay content of the recycled water can reduce primary froth yield by increasing the viscosity of the middlings layer and retarding the upward settling of oil flecks. When this occurs, the smaller oil flecks and those that are more heavily laden with mineral matter stay suspended in the water of the separation cell and are removed from the cell with the middlings layer.
Waste water streams discharged from the hot water process for extracting bitumen from tar sands often called effluent discharge contain a substantial amount of mineral matter, much of which is colloidally dispersed in the effluent discharge and therefore does not settle very readily when stored in the retention pond. The lower layer of the retention pond can contain up to 50% dispersed mineral matter comprised substantially of clay and silt as well as up to 25% bitumen. This part of the pond water is normally referred to as sludge. Sludge is not suitable for recycling to the hot water extraction process for the reason that its addition into the separation cell or the scavenger cell at the normal inlet means would raise the mineral content of the middlings of the cell to the extend that recovery of bitumen would be substantially reduced. Generally, the settling which does take place in the pond provides a body of water in which the concentration of mineral matter increases substantially from the surface of the pond to the bottom thereof.
A waste water retention pond of the type herein described is normally formed over a reasonably long period of time. A hot water extraction plant for recovering bitumen from tar sands can produce between 12,000 and 25,000 imperial gallons per minute (IGPM) of waste water streams which are stored in the pond. Concurrently, of course, some of the pond water, i.e., that containing less than 5% mineral matter, can be recycled to the hot water extraction process. Recycling pond water serves to reduce the overall volume increase of water stored in the retention pond.
Experience has shown that, as the pond forms, the various components in the effluent discharge settle in the pond at varying rates. As an example, when the waste water containing sand, silt, clay and bitumen is discahrged to the pond, the fee bitumen normally immediately floats to the surface of the pond and the sand immediately settles to the bottom of the pond. However, after the surface bitumen cools and releases the entrapped air which originally caused it to float, it too will begin to settle toward the bottom of the pond. The silt and clay in the discharge settle in the pond at a substantially low rate as compared to the sand.
Thus to characterize a pond, it can be pictured as a large body of water containing dispersed solids which are slowly settling toward the bottom of the pond. The mineral matter in the pond is in a constant but slow state of settling. Normally, the pond is constantly increasing in size because of the continuous addition of waste water and therefore the character of the pond is continually changing.
In processing tar sands to recover butumen therefrom, the tar sands are excavated, extracted to remove the bitumen, whereafter the sand and other minerals are returned to the excavated area. As noted above, waste waters associated with the extraction step must be stored in a retention pond which is normally placed in one of the excavated areas. It is important that the excavated area be filled only with minerals and not with water since obviously the water is excess and therefore requires more storage volume than is available. If a retention pond associated with the hot water extraction of bitumen from tar sands is not treated to remove water layers which cannot normally be reused, such as sludge, the problem of a shortage of storage space is ever present.
As one example, a waste water retention pond associated with hot water process for extracting bitumen from 140,000 to 150,000 tons of tar sands per day and having a surface area of about 1,000 acres and an avarage depth of 40 feet can be characterized somewhat as follows:
a. From the surface of the pond to a depth of about 15 feet the mineral concentration which is primarily clay is found to be about 0.5 to 5.0 weight percent. This pond water can normally be recycled to a hot water extraction process without interfering with the extraction of bitumen from tar sands.
b. The layer of water in the pond between 15 and 25 feet from the surface contains between 5.0 and 20% mineral matter. This water, if recycled to the separation cell feed with fresh tar sands, would increase the mineral content of the middlings portion of the cell to the point that little bitumen would be recovered.
c. Finally, the section of the pond between 25 feet and the bottom of the pond contains 20 to 50% mineral matter and is normally referred to as sludge.
Many procedures for treating waste waters associated with the extraction of bitumen from tar sands have been proposed. For example, Canadian Pat. No. 841,582 issued May 12, 1970 to R. A. Baillie claims a method for recovering additional bitumen from waste water streams recovered from a tar sands hot water extraction process comprising settling the stream and removing floating bitumen from the surface thereof.
Canadian Pat. No. 824,968 issued Oct. 14, 1969 to Robert A. Baillie discloses a treatment of waste water from a hot water extraction process which comprises percolating the waste water through an inclined sand pile to incorporate the clay and silt of the waste water into the interstices of the sand pile.
Canadian Pat. No. 866,266 issued Mar. 16, 1971 to Raymond et al. discloses removing bitumen from waste water streams by incorporating viable microorganisms therein which subsequently results in clay settling. Canadian Pat. Nos. 873,317 issued June 16, 1971 to Baillie et al.; 873,318 issued June 16, 1971 to Baillie et al.; 873,853 issued June 22, 1971 to Baillie et al.; 874,418 issued June 29, 1971 to Camp; 874,419 issued June 29, 1971 to Steinmetz, 878,656 issued Aug. 17, 1971 to Seitzer et al.; 882,668 issued Oct. 5, 1971 to Camp; 890,804 issued Jan. 18, 1972 to Fear et al.; 891,472 issued Jan. 25, 1972 to Camp; 892,548 issued Feb. 8, 1972 to Hepp et al. and 917,586 issued Dec. 26, 1972 to Paulson each disclose methods for treating waste water streams associated with the hot water method for extracting bitumen from tar sands. Yet none of these proposals provides an economically attractive process for treating hot water extraction process waste waters associated with the recovery of bitumen from tar sands. By the method of the present invention an improved process for resolving this problem is provided.